A styrene-butadiene-styrene triple block copolymer composed of a vinyl aromatic hydrocarbon block and a conjugated diene block is prepared by a solution polymerization using an anion initiator. Due to excessive exothermic reaction, a maximum polymerization temperature reaches 100° C. or higher.
During anion polymerization, the styrene-butadiene-styrene triple block copolymer is stabilized over a considerably long time, in particular, one week, at room temperature in the form of an anion on polymer chain ends immediately before the terminal reaction after growth reaction, but hydrogen anion separation reaction is caused by heat at a high temperature of 100° C. or higher and side reaction in which the terminal anions form unsaturated double bonds thus occurs. These side reactions are inevitable, although the time at which immediately before terminal reaction after growth reaction is taken is within several minutes.
Such side reactions cause deterioration in activity of an end of polymer chain and thus deterioration in coupling efficiency during coupling polymerization, cause reactions between anion ends of living polymers and polymers produced by side reactions, thus producing polymers with a doubled molecular weight. At this time, 10 to 15% by weight of the obtained polymer is generally produced. Furthermore, even during identical polymerization, since the content of polymers with a high molecular weight is not uniform, process control is not easy in products requiring accurate control of molecular weight and product defect rates thus increase.
In terms of physical properties, these side products improve physical properties of obtained products, but solution polymerization should be performed at low temperatures and at a low solid content due to deterioration in processability and limited control in the polymerization process, thus causing a deterioration in commercial applicability of products. Accordingly, there is a need for a method that can stabilize anion ends although polymerization is performed at a high temperature in order to secure commercial potential and maximize coupling efficiency through inhibition of production of side reactions and accurately control molecular weight.